1. The Field of the Invention
The present invention relates to methods for purifying electrically conductive polymers. More particularly, the present invention relates to methods for purifying conductive polymers using a solid scavenger.
2. The Relevant Technology
Conductive polymers possess conjugated π-electron systems along their backbone, thereby giving them the ability to support positive and negative charge carries with controllable mobility along the chains. In addition to the metallic and semiconductor characteristics, conductive polymers also exhibit unique electroluminescence properties. These features of conductive polymers have generated strong interest from academia and industry researchers. However, the use of conductive polymers in commercial products has been rather limited so far.
One reason for the lack of commercialization of conductive polymers is due to the difficulty of adequately purifying the polymers. Even small amounts of impurities in the polymers may act as charge trappers or photo quenchers, and alter the polymer's semiconductor characteristics.
In light emitting devices fabricated from conductive polymers, metallic impurities can act as recombination centers for injected charge carries, which leads to an increase in recombination rates. The increased recombination rates often degrade the light emitting efficiency and increase the current leakage of the devices. Another problem with contamination is variable semiconducting properties. Doping concentrations in semiconductors are controlled to achieve desired semiconductor properties. If the metal content in a given electronic polymer system is not controllable, its semi-conductive behavior will not be predictable.
Despite the need to limit metal impurities, most polymer synthesis techniques use relatively large amounts of transition metal catalysts. The use of a catalyst in polymer synthesis is required to obtain reasonable reaction rates for commercial production of polymer.
Recently attempts have been made to purify conductive polymers using metal scavengers. The metal scavengers bind to the metal impurities and can then be separated from the polymer using filtration. While this technique appears to be promising, its implementation has been difficult. One problem with the scavenger technique has been low yields. The amount of impurity removed for a given amount of scavenger is low, which means that excess scavenger must be used to achieve desired results.
Another problem with some existing scavengers is the cost of manufacturing the scavengers. The metal scavengers are typically inorganic particles such as silica that are functionalized with a silane. Silanes are relatively expensive for use as scavengers. The costs associated with silanes contribute to the expenses that make commercial production of conductive polymers less feasible.
Finally despite recent improvements in polymer purification techniques, there is still a need for further improvements in purification. Additional purity is needed to expand the life of the polymers and enable their use in a wider variety of applications.